US20160188949A1

US20160188949A1 - Fingerprint sensing device and fingerprint sensing method thereof

Info

Publication number: US20160188949A1
Application number: US14/978,599
Authority: US
Inventors: Chao-Chi Yang
Original assignee: Elan Microelectronics Corp
Current assignee: Elan Microelectronics Corp
Priority date: 2014-12-26
Filing date: 2015-12-22
Publication date: 2016-06-30
Also published as: CN105740756A

Abstract

A fingerprint sensing device comprises a shielding plate configured between an electrode plate and a detection circuit for reducing a parasitic capacitor between the electrode plate and a conductor thereunder. Consequently, a larger signal dynamic range can be achieved and the electrode plate can be prevented from operation noise interference of the detection circuit. The shielding plate and the electrode plate have the same potential. Accordingly, a parasitic capacitor effect between the shielding plate and the electrode plate can be eliminated. Thus, the fingerprint sensing device of the present invention has a better noise resistibility.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit of U.S. Provisional Patent Application Ser. No. 62/096,894, filed Dec. 26, 2014, and Taiwan Patent Application No. 104139496, filed Nov. 26, 2015, which are hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention is related generally to a fingerprint sensing device and a method thereof, more particularly, to a low parasitic capacitance fingerprint sensing device and a fingerprint sensing method thereof.

BACKGROUND OF THE INVENTION

FIG. 1 shows a conventional fingerprint sensing device 10. A protection layer 12 is provided for fingers to touch and for protecting electrode plates 16 a, 16 b, and 16 c thereunder. An electro-static discharge (ESD) layer 14 provides an ESD protection. Detection circuits 18 a, 18 b, and 18 c are connected to the electrode plates 16 a, 16 b, and 16 c, respectively, thereby detecting a capacitance value between the electrode plates 16 a, 16 b, and 16 c and a finger (not shown) so as to acquire a sensing voltage. Wherein, fingerprints of the finger consist of uneven lines. Thus, the fingerprints have peaks and valleys. Moreover, a distance between the peak of the fingerprint and the electrode plate is different from a distance between the valley of the fingerprint and the electrode plate, which also generates different sensing voltages. The fingerprint sensing device 10 judges the lines above the electrode plates 16 a, 16 b, and 16 c are peaks or valleys according to the sensing voltages. After the fingerprint sensing device 10 acquires all the lines above the electrode plates, a fingerprint image of fingers can be acquired.
However, as shown by FIG. 1, there are parasitic capacitors Cp1 a, Cp1 b, and Cp1 c between the electrode plates 16 a, 16 b, and 16 c and conductors thereunder. The conductors under the electrode plates include detection circuits 18 a, 18 b, and 18 c, a ground terminal, and other conductors. The parasitic capacitors Cp1 a, Cp1 b, and Cp1 c will influence the sensing of the electrode plates 16 a, 16 b, and 16 c. The larger the parasitic capacitors Cp1 a, Cp1 b, and Cp1 c are, the smaller a dynamic range of the sensing voltage which is generated by measuring the electrode plates 16 a, 16 b, and 16 c will be. Consequently, it will be more difficult to correctly judge that the line is peak or valley. Further, operation noise of the detection circuits 18 a, 18 b, and 18 c also interferes with the electrode plates 16 a, 16 b, and 16 c via the parasitic capacitor Cp1 a, Cp1 b, and Cp1 c.
Therefore, it is desired a low parasitic capacitance fingerprint sensing device.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a low parasitic capacitance fingerprint sensing device and a fingerprint sensing method thereof.
According to the present invention, a fingerprint sensing device comprises an electrode plate, a feedback capacitor, a component layer, and a shielding plate. The feedback capacitor is coupled to the electrode plate. The feedback capacitor and the electrode plate are independent components. The component layer is under the electrode plate. The component layer includes several circuit components connected to the feedback capacitor so as to form a detection circuit for detecting a capacitance value between a finger and the electrode plate. Accordingly, a fingerprint above the electrode plate can be judged by the capacitance value. The shielding plate is configured between the electrode plate and the component layer. In an exciting mode, a first voltage is provided to the electrode plate and the shielding plate. In a detecting mode, a second voltage is provided to the electrode plate and the shielding plate.
According to the present invention, a fingerprint sensing device comprises an electrode plate, a detection circuit, a first switch, and a shielding plate. In a detecting mode, the detection circuit detects a capacitance value between the electrode plate and a finger, wherein the capacitance value is used to judge a fingerprint above the electrode plate. The first switch is connected between the electrode plate and the detection circuit. The shielding plate is configured between the electrode plate and the detection circuit. In an exciting mode, the first switch is open so as to disconnect a connection between the electrode plate and the detection circuit and a first voltage is provided to the electrode plate and the shielding plate. In a detecting mode, the first switch is closed so as to make the detection circuit to connect the electrode plate and a second voltage is provided to the electrode plate and the shielding plate.
According to the present invention, a method for sensing fingerprints comprises the steps of: in an exciting mode, disconnecting a connection between an electrode plate and a detection circuit and providing a first voltage to the electrode plate and a shielding plate; and in a detecting mode, connecting the electrode plate to the detection circuit, providing a second voltage to the electrode plate and the shielding plate, detecting the capacitance value between a finger and the electrode plate by the detection circuit and judging a fingerprint above the electrode plate by the capacitance value. Wherein, the shielding plate is configured between the electrode plate and the detection circuit.
The present invention uses the shielding plate under the electrode plate to reduce the parasitic capacitor between the electrode plate and other conductors thereunder. Accordingly, a larger signal dynamic range can be achieved and the electrode plate can be prevented from operation noise interference of the detection circuit. Besides, since the shielding plate and the electrode plate have the same potential during the sensing, the parasitic capacitance effect between the shielding plate and the electrode plate can be eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments according to the present invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a conventional fingerprint sensing device;

FIG. 2 shows a first embodiment of a fingerprint sensing device of the present invention;

FIG. 3 shows a structure of the fingerprint sensing device in FIG. 2;

FIG. 4 shows an equivalent circuit of the fingerprint sensing device in FIG. 2 under an exciting mode;

FIG. 5 shows the equivalent circuit of the fingerprint sensing device in FIG. 2 under a detecting mode;

FIG. 6 shows timing diagrams of circuits in FIGS. 4 and 5; and

FIG. 7 shows a second embodiment of the fingerprint sensing device of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows an embodiment of a fingerprint sensing device 22 of the present invention, which comprises the similar protection layer 12, ESD layer 14, electrode plate 16 a, 16 b, and 16 c, and detection circuits 18 a, 18 b, and 18 c as those in the conventional fingerprint sensing device 10 in FIG. 1. Besides, the fingerprint sensing device 22 further comprises shielding plates 24 a, 24 b, and 24 c, and switches SWse and SWsp. Wherein, the shielding plate 24 a is configured between the electrode plate 16 a and the detection circuit 18 a. The shielding plate 24 b is configured between the electrode plate 16 b and the detection circuit 18 b. The shielding plate 24 c is configured between the electrode plate 16 c and the detection circuit 18 c. One terminal of the switch SWse is connected to the shielding plates 24 a, 24 b, and 24 c. The other terminal of the switch SWse receives a voltage VR1. One terminal of the switch SWsp is connected to the shielding plates 24 a, 24 b, and 24 c. The other terminal of the switch SWsp receives a voltage VR2. In the fingerprint sensing device 22, the shielding plates 24 a, 24 b, and 24 c can reduce the parasitic capacitors between the electrode plates 16 a, 16 b, and 16 c and other conductors (such as the detection circuits 18 a, 18 b, and 18 c, and a ground) thereunder from Cp1 a, Cp1 b, and Cp1 c in FIG. 1 to Cp1 aa, Cp1 ba, and Cp1 ca. Wherein, the parasitic capacitors Cp1 aa, Cp1 ba, and Cp1 ca are much smaller than the parasitic capacitors Cp1 a, Cp1 b, and Cp1 c. Since the parasitic capacitors correspondent to the electrode plates 16 a, 16 b, and 16 c are quite small, a larger signal dynamic range can be achieved to acquire a larger signal amount. Further, the shielding plates 24 a, 24 b, and 24 c can lessen that the operation noise of the detection circuits 18 a, 18 b, and 18 c interfere with the electrode plates 16 a, 16 b, and 16 c. Moreover, by switching the switches SWse and SWsp, the shielding plates 24 a, 24 b, and 24 c and the electrode plates 16 a, 16 b, and 16 c have the same potential. As a result, during sensing the electrode plates 16 a, 16 b, and 16 c, the effects of the parasitic capacitor Cp1 ab, Cp1 bb, and Cp1 cb between the shielding plates 24 a, 24 b, and 24 c and the electrode plates 16 a, 16 b, and 16 c can be eliminated.
FIG. 3 shows an embodiment of a structure of the fingerprint sensing device 22 in FIG. 2. In this embodiment, for convenient illustration, only one sensing unit is shown. The sensing unit includes the electrode plate 16 a, the detection circuit 18 a, the shielding plate 24 a, and switches SW1 a, SW2 a, SWse, and SWsp. In FIG. 3, switches SW1 a, SW2 a, SW3 a, SWse, and SWsp and an operation amplifier 20 a are disposed in a component layer 32. Wherein, the operation amplifier 20 a, the switch SW3 a and a feedback capacitor Cfba form the detection circuit 18 a. The feedback capacitor Cfba is disposed between the shielding plate 24 a and the component layer 32 and right under the shielding plate 24 a. The feedback capacitor Cfba consists of metal plates 28 and 30, and the feedback capacitor Cfba and the electrode plate 16 a are independent components. The shielding plate 24 a is configured between the electrode plate 16 a and the component layer 32. The shielding plate 24 a is disposed right under the electrode plate 16 a so as to reduce the parasitic capacitor between electrode plate 16 a and other conductors (such as the detection circuit 18 a and the ground) thereunder. The component layer 32 further includes a semi-conductor substrate (not shown) for manufacturing the components of the detection circuit 18 a. The feedback capacitor Cfba can be formed by other means. For example, the feedback capacitor Cfba can be formed by two layers of Polysilicon in the component layer 32.
When a finger 34 touches the fingerprint sensing device 22, a capacitor Csa will be generated between the finger 34 and the electrode plate 16 a. Accordingly, detecting the capacitor Csa can judge that a line of a fingerprint above the electrode plate 16 a is peak or valley. In an exciting mode, switches SW1 a, SW3 a, and SWsp are closed (on), and switches SW2 a and SWse are open (off). At this time, the voltage VR2 is provided to the electrode plate 16 a and the shielding plate 24 a, and the feedback capacitor Cfba is in a short circuit state. Therefore, the voltage on the feedback capacitor Cfba is set as 0V. In a detecting mode, switches SW1 a, SW3 a and SWsp are open, and switches SW2 a and SWse are closed. Accordingly, the electrode plate 16 a and the shielding plate 24 a are connected to an invert input terminal of an operation amplifier 20 a and the voltage VR1, respectively. Since the operation amplifier has a characteristic of virtual ground, the voltage VR1 is also provided to the electrode plate 16 a. In the meantime, the detection circuit 18 a detects the capacitor Csa to generate a sensing voltage Voa to judge that the line above the electrode plate 16 a is peak or valley. In both the exciting mode and the detecting mode, the potentials of the electrode plate 16 a and the shielding plate 24 a are the same. Consequently, the effect of the parasitic capacitor Cp1 ab between the electrode plate 16 a and the shielding plate 24 a will be eliminated.
FIGS. 4 and 5 show equivalent circuits of the fingerprint sensing device 22 in FIG. 2. Wherein, FIG. 4 shows an operation in the exciting mode, and FIG. 5 shows an operation in the detecting mode. In FIGS. 4 and 5, Csa, Csb, Csc, and Csd are capacitors formed by a finger and the electrode plates 16 a, 16 b, 16 c, and 16 d. The electrode plates 16 a, 16 b, 16 c, and 16 d are regarded as the right electrodes of the capacitors Csa, Csb, Csc, and Csd, respectively. The finger is regarded as the left electrodes of the capacitors Csa, Csb, Csc, and Csd. One terminal of the switch SW is connected to the electrode plate 16 a and the switch SW2 a. The other terminal of the switch SW1 a receives the voltage VR2. The switch SW2 a is connected between the electrode plate 16 a and the detection circuit 18 a. Cp1 aa represents the parasitic capacitor between the electrode plate 16 a and the conductors thereunder. Cp1 ab is the parasitic capacitor between the electrode plate 16 a and the shielding plate 24 a. The detection circuit 18 a includes the operation amplifier 20 a, the switch SW3 a, and the feedback capacitor Cfba. Wherein, the switch SW3 a and the feedback capacitor Cfba are a parallel connection between an invert input terminal Ina and an output terminal Oa of the operation amplifier 20 a. A non-invert input terminal Ipa of the operation amplifier 20 a receives the voltage VR1. The capacitor Cp2 a represents the parasitic capacitor of the invert input terminal Ina of the operation amplifier 20 a. One terminal of a switch SW1 b is connected to the electrode plate 16 b and a switch SW2 b. The other terminal of the switch SW receives the voltage VR2. The switch SW2 b is connected between the electrode plate 16 b and the detection circuit 18 b. Cp1 ba represents the parasitic capacitor between the electrode plate 16 b and the conductors thereunder. Cp1 bb is the parasitic capacitor between the electrode plate 16 b and the shielding plate 24 b. The detection circuit 18 b includes an operation amplifier 20 b, a switch SW3 b, and a feedback capacitor Cfbb. Wherein, the switch SW3 b and the feedback capacitor Cfbb are a parallel connection between an invert input terminal Inb and an output terminal Ob of the operation amplifier 20 b. A non-invert input terminal Ipb of the operation amplifier 20 b receives the voltage VR1. The capacitor Cp2 b represents is the parasitic capacitor of the invert input terminal Inb of the operation amplifier 20 b. One terminal of a switch SW1 c is connected to the electrode plate 16 c and the switch SW2 c. The other terminal of the switch SW1 c receives the voltage VR2. The switch SW2 c is connected between the electrode plate 16 c and the detection circuit 18 c. Cp1 ca represents the parasitic capacitor between the electrode plate 16 c and the conductors thereunder. Cp1 cb is the parasitic capacitor between the electrode plate 16 c and the shielding plate 24 c. The detection circuit 18 c includes an operation amplifier 20 c, a switch SW3 c, and a feedback capacitor Cfbc. Wherein, the switch SW3 c and the feedback capacitor Cfbc are a parallel connection between an invert input terminal Inc and an output terminal Oc of the operation amplifier 20 c. A non-invert input terminal Ipc of the operation amplifier 20 c receives the voltage VR1. The capacitor Cp2 c represents the parasitic capacitor of the invert input terminal Inc of the operation amplifier 20 c. One terminal of a switch SW1 d is connected to the electrode plate 16 d and a switch SW2 d. The other terminal of the switch SW1 d receives the voltage VR2. The switch SW2 d is connected between the electrode plate 16 d and the detection circuit 18 d. Cp1 da represents the parasitic capacitor between the electrode plate 16 d and the conductors thereunder. Cp1 db is the parasitic capacitor between the electrode plate 16 d and the shielding plate 24 d. The detection circuit 18 d includes an operation amplifier 20 d, a switch SW3 d, and a feedback capacitor Cfbd. Wherein, the switch SW3 d and the feedback capacitor Cfbd are a parallel connection between an invert input terminal Ind and an output Od of the operation amplifier 20 d. A non-invert input terminal Ipd of the operation amplifier 20 d receives the voltage VR1. The capacitor Cp2 d represents the parasitic capacitor of the invert input terminal Ind of the operation amplifier 20 d. In FIGS. 4 and 5, there are the shielding plates 24 a, 24 b, 24 c, and 24 d between the electrode plates 16 a, 16 b, 16 c, and 16 d and the detection circuits 18 a, 18 b, 18 c, and 18 d, so the parasitic capacitors between the electrode plates 16 a, 16 b, 16 c, and 16 d and other conductors thereunder are lowering from Cp1 a, Cp1 b, and Cp1 c in FIG. 1 to Cp1 aa, Cp1 ba, Cp1 ca, and Cp1 da. The parasitic capacitors Cp1 aa, Cp1 ba, Cp1 ca, and Cp1 da are much smaller than the parasitic capacitors Cp1 a, Cp1 b, and Cp1 c.
FIG. 6 shows timing diagrams of the circuits under detecting the electrode plate 16 a in FIGS. 4 and 5. As shown by the circuits in FIG. 4 and time t1˜t2 in FIG. 6, when the fingerprint sensing device 22 is in the exciting mode, switches SW1 a, SW3 a, SW1 b, SW3 b, SW1 c, SW3 c, SW1 d, SW3 d, and SWsp are closed (on), and switches SW2 a, SW2 b, SW2 c, SW2 d, and SWse are open (off). In the meantime, the voltage VR2 charges the capacitors Csa, Csb, Csc, and Csd, and the voltages of the feedback capacitors Cfba, Cfbb, Cfbc, and Cfbd are 0V. Since the operation amplifier has a characteristic of virtual ground, the voltages of the invert input terminals Ina, Inb, Inc, and Ind of the operation amplifiers 20 a, 20 b, 20 c, and 20 d equal VR1. The output terminals Oa, Ob, Oc, and Od of the operation amplifiers 20 a, 20 b, 20 c, and 20 d are connected to the invert input terminals Ina, Inb, Inc, and Ind, so sensing voltages Voa, Vob, Voc, and Vod will equal VR1. In the exciting mode, the potentials at two terminals of the parasitic capacitors Cp1 ab, Cp1 bb, Cp1 cb, and Cp1 db are VR2. Thus, the voltages of the parasitic capacitors Cp1 ab, Cp1 bb, Cp1 cb, and Cp1 db are 0V. When the exciting mode ends as shown by the time t2 in FIG. 6, the switches SW1 a, SW3 a, SW1 b, SW1 c, SW1 d, and SWsp are became open. Switches SW2 a, SW2 b, SW2 c, SW2 d, and SWse are kept open. Switches SW3 b, SW3 c, and SW3 d are kept closed.
As shown by the circuits in FIG. 5 and time t3˜t4 in FIG. 6, when the fingerprint sensing device 22 enters the detecting mode for detecting the fingerprint corresponding to the electrode plate 16 a, switches SW1 a, SW3 a, SW1 b, SW1 c, SW1 d, and SWsp are kept open. Switches SW2 a, SW2 b, SW2 c, SW2 d, and SWse are became closed. Switches SW3 b, SW3 c, and SW3 d are kept closed. In the meantime, the sensing voltage Voa=VR1−(VR2−VR1)×[(Csa/Cfba)+(Cp1 aa/Cfba)]. The fingerprint sensing device 22 determines the capacitance value of the capacitor Csa according to the sensing voltage Voa so as to judge that the line corresponding to the electrode plate 16 a is peak or valley. When the detecting mode ends as shown by time t4 in FIG. 6, switches SW1 a, SW3 a, SW1 b, SW1 c, SW1 d, and SWsp are kept open. Switches SW2 a, SW2 b, SW2 c, SW2 d, and SWse are became open. Switches SW3 b, SW3 c, and SW3 d are kept closed. In the detecting mode, due to the virtual ground of the operation amplifier, the potentials at two terminals of the parasitic capacitors Cp1 ab, Cp1 bb, Cp1 cb, and Cp1 db are VR1. From aforementioned equation of the sensing voltage Voa, the parasitic capacitor Cp1 ab between the electrode plate 16 a and the shielding plate 24 a does not influence the sensing voltage Voa. Moreover, there is only the very small parasitic capacitor Cp1 aa between the electrode plate 16 a and the conductors thereunder. Thus, the fingerprint sensing device 22 of the present invention has a larger signal dynamic range than that of the conventional fingerprint sensing device 10 with the larger parasitic capacitor Cp1 a. As a result, the present invention provides a larger output signal amount. Additionally, the present invention also can lessen that the operation noises of the detection circuits influence the electrode plates. Further, the switches in the figures can be configured under the electrode plates and the shielding plates. Namely, the present invention also lessen that the operation noises of the switches influence the electrode plates.
In FIG. 6, during the process from the exciting mode to the detecting mode, switches SW1 a, SW3 a, SW1 b, SW1 c, SW1 d, and SWsp will be opened before switches SW2 a, SW2 b, SW2 c, SW2 d, and SWse are closed.
FIG. 5 shows an example of measuring the capacitor Csa between the electrode plate 16 a and the finger. People skilled in the art know how to measure the other electrode plates properly, which will be hereby omitted.
In the fingerprint sensing device 22 in FIG. 2, each of the electrode plates 16 a, 16 b, and 16 c is corresponding to each of the detection circuits 18 a, 18 b, and 18 c, respectively. In other embodiments, the electrode plates 16 a, 16 b, and 16 c can share one detection circuit 18 a as shown by FIG. 7. In FIG. 7, a switching circuit 36 is connecting the detection circuit 18 a to the detected electrode plates 16 a, 16 b, or 16 c.
In the embodiments in FIGS. 2, 4, 5, and 7, the shielding plates 24 a, 24 b, 24 c, and 24 d share the switches SWse and SWsp. In other embodiments, one shielding plate can correspond to one set of switches SWse and SWsp.
While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.

Claims

What is claimed is:

1. A fingerprint sensing device, comprising:

an electrode plate;

a feedback capacitor coupled to the electrode plate, wherein the feedback capacitor and the electrode plate are independent components;

a component layer under the electrode plate including several circuit components that are connected to the feedback capacitor, wherein the several circuit components and the feedback capacitor form a detection circuit for detecting a capacitance value between a finger and the electrode plate, and the capacitance value is used to judge a fingerprint above the electrode plate; and

a shielding plate configured between the electrode plate and the component layer;

wherein, in an exciting mode, a first voltage is provided to the electrode plate and the shielding plate and in a detecting mode, a second voltage is provided to the electrode plate and the shielding plate.

2. The fingerprint sensing device of claim 1, wherein the several circuit components comprise:

an operation amplifier having an invert input terminal, a non-invert input terminal which receives the second voltage, and an output terminal; and

a first switch connected to the feedback capacitor in parallel between the invert input terminal and the output terminal of the operation amplifier, wherein in the exciting mode, the first switch is closed and in the detecting mode, the first switch is open.

3. The fingerprint sensing device of claim 2, wherein the component layer further comprises:

a second switch having one terminal connected to the invert input terminal of the operation amplifier and the other terminal connected to the electrode plate, wherein in the exciting mode, the second switch is open and in the detecting mode, the second switch is closed;

a third switch having one terminal connected to the second switch and the electrode plate and the other terminal for receiving the first voltage, wherein in the exciting mode, the third switch is closed and in the detecting mode, the third switch is open;

a fourth switch having one terminal connected to the shielding plate and the other terminal for receiving the first voltage, wherein in the exciting mode, the fourth switch is closed and in the detecting mode, the fourth switch is open; and

a fifth switch having one terminal connected to the shielding plate and the other terminal for receiving the second voltage, wherein in the exciting mode, the fifth switch is open, and in the detecting mode, the fifth switch is closed.

4. A fingerprint sensing device, comprising:

an electrode plate;

a detection circuit for detecting a capacitance value between the electrode plate and a finger in a detecting mode, wherein the capacitance value is used to judge a fingerprint above the electrode plate;

a first switch connected between the electrode plate and the detection circuit, wherein in an exciting mode, the first switch is open for disconnecting a connection between the electrode plate and the detection circuit and in a detecting mode, the first switch is closed for connecting the detection circuit to the electrode plate; and

a shielding plate configured between the electrode plate and the detection circuit;

wherein, in the exciting mode, a first voltage is provided to the electrode plate and the shielding plate and in the detecting mode, a second voltage is provided to the electrode plate and the shielding plate.

5. The fingerprint sensing device of claim 4, wherein the detection circuit comprises:

an operation amplifier having an invert input terminal, a non-invert input terminal which receives the second voltage, and an output terminal;

a feedback capacitor connected between the invert input terminal and the output terminal;

a second switch connected to the feedback capacitor in parallel, wherein in the exciting mode, the second switch is closed and in the detecting mode, the second switch is open.

6. The fingerprint sensing device of claim 5, further comprising:

a third switch having one terminal connected to the first switch and the electrode plate and the other terminal for receiving the first voltage, wherein in the exciting mode, the third switch is closed and in the detecting mode, the third switch is open;

7. A method for sensing fingerprints, comprising the steps of:

in an exciting mode, disconnecting a connection between an electrode plate and a detection circuit and providing a first voltage to the electrode plate and a shielding plate; and

in a detecting mode, connecting the electrode plate to the detection circuit and providing a second voltage to the electrode plate and the shielding plate, wherein the detection circuit detecting a capacitance value between a finger and the electrode plate and the capacitance value is used to judge that a fingerprint above the electrode plate;

wherein, the shielding plate is configured between the electrode plate and the detection circuit.

8. The method of claim 7, further comprising the steps of:

setting a voltage of a feedback capacitor connected between an invert input terminal and an output terminal of an operation amplifier in the detection circuit in an exciting mode; and

connecting the feedback capacitor to the electrode plate in a detecting mode so as to make the feedback capacitor to generate a sensing voltage for judging the fingerprint above the electrode plate, wherein the sensing voltage is related to the capacitance value between the finger and the electrode plate.